In order to overcome this difficulty and thus minimize this extraneous flux, there have been suggestions, for instance, to cover certain internal components in the cryostat with a material that absorbs the wavelengths of the stray infrared radiation at angles of incidence that are as high as possible in relation to normal to the surface.
There have also been suggestions to fit, at the level of the internal face of the cold shield, a certain number of radially directed baffles whose function is to stop or outwardly reflect incident rays in a direction other than that of the detector's nominal field of view.
The drawback of these baffles is the fact that they are fitted on the internal face of the cold shield by bonding, especially by using an epoxy resin. Apart from the fact that fixing the baffles involves a time-consuming and awkward operation that makes it more expensive to produce such a detector, experience shows that the epoxy resin used to ensure this bonding causes outgassing which, sooner or later, affects the efficiency of the detector which is generally vacuum sealed.
FIG. 1 schematically shows a quantum infrared detector, i.e. a quantum infrared detector of a type that is known in itself and which is designed to operate at low temperature.
Basically, it comprises a cryostat or chamber 1 whereof the cold finger 2 is connected to a cold source (not shown). The upper part of this cryostat is closed by a window 3 that is transparent to the infrared radiation that is to be detected.
A cold plane 4 which is in heat exchange with cold finger 2 is mechanically fixed to the upper end of the cold finger 2 of cryostat 1. Reference 5 denotes the infrared detector unit which is itself in heat exchange with cold plane 4. This detector unit typically consists of a plurality of elementary detectors associated with an evaluation circuit capable of converting the signals output by these detectors into usable electrical signals. The resulting information is transmitted to the cryostat's external environment by connections, especially wired connections, which are not shown in order not to include unnecessary details in the figure.
A cold shield 6 which also comprises an opening 7 that faces window 3, which is transparent to the infrared radiation that is to be detected, in the upper end of the cryostat is also mounted on cold plane 4. This opening 7 acts as a field stop.
This diagram shows, firstly, by an arrow that is substantially perpendicular to the detector unit 5, the wanted infrared radiation located in the detector's field of view and, secondly, by a diagonal arrow that is reflected on the side wall of cold shield 6, stray radiation that is not in the detector's field of view and which is therefore to be eliminated.
FIG. 2 shows a cold shield according to the prior art which uses radial baffles 8. These metal baffles are generally of the same type as the cold shield, are parallel to each other and are fitted on the internal face 10 of the cold shield. The latter is a rotationally symmetrical type cold shield. It defines a surface 9 at its base which substantially corresponds to the useful surface area of the detector, i.e. the surface area of the material that is sensitive to the infrared radiation that is to be detected.